The invention relates to an electric rain sensor for mounting on the outside of the main frame or on another stationary member of an openable window, in particular a skylight, of the type where opening and closing may be carried out by means of an electrical maneuvering device, said rain sensor comprising a sensing element with two electrodes opposite one another in or on a substantially plane sensing surface of an insulating holder, the distance between the electrodes being less than the size of a raindrop, means for applying a voltage between the electrodes and an electrical circuit connected to the electrodes for detecting a change in an electrical quantity caused by the presence of raindrops, a heating element being mounted on the holder for drying of the sensing surface, and the holder being produced by molding of an electrically insulating thermoplastic.
Such rain sensors are known for instance from DE-A-2363978, DE-A-4023673, and DE-A-3939317 in addition to CA-A-1119669 and are often installed in connection with electrically operated windows in order to ensure the closing of an open skylight in the event of rain or snow. In known installations the sensing element is frequently positioned inside on the main frame, in order to avoid exposure to bad weather resulting in contamination and corrosion to such a degree that the sensor no longer functions as intended. Positioning the sensing element on the inside, however, often results in a particular drawback, namely that it frequently is placed in such a manner--for instance on a vertical section of the main frame--as to be sheltered from the wind when the wind is in certain directions thereby being struck by the rain either too late or not at all.
DE-C-3 246 412 discloses a humidity sensing element for industrial application, wherein the electrodes are imbedded in an added insulating layer.
With some types of rain sensors a relatively long period of time may pass before the sensing surface--after having been exposed to rain--dries and consequently the window can only be opened after a significant time delay with respect to the end of the rainfall.
It has been attempted to avoid these problems by the positioning of a rain sensor of the kind mentioned above on the outside and attaching an electrical heating element to the sensor, the heating element being either permanently activated or turned on immediately after the sensor has become wet.